Pollutants tend to build up on such surfaces when there is no rain to wash them away, and when rain finally comes, it washes a higher concentration of the toxic substances into urban streams, which normally already carry high amounts of pollutants.

"Young salmon in hatcheries like this are like canaries in a mine," Murdoch said. "When those guys go belly up, you've got problems."

A half-dozen cutthroat trout also were killed, a major blow for tiny Willow Creek, which was just seeing the species begin to rebound in numbers, said Walt Thompson, manager of the hatchery.

About 500 hatchery salmon survived, but those won't be released for several weeks to make sure they have a good chance of survival.

The hatchery had planned to hold an educational open house as part of the Edmonds Waterfront Festival Thursday through Sunday, but that event will likely be canceled.